Under nominal internal combustion engine operating conditions, the temperatures of the exhaust gases leaving the tail pipe are higher than the ambient temperature. This condition is expected due to the combustion event which occurs in the engine. With the addition of aftertreatment systems, the exhaust temperatures must be increased periodically above the exhaust temperatures resulting from nominal engine operations to help maintain the proper operation and efficiency of the aftertreatment system. The duration and increase in temperature above nominal depends on the type of aftertreatment device, the materials used in the construction of the specific aftertreatment device, and the type and amount of contaminants to be removed, among other factors.
The increase in exhaust outlet temperature can impact objects located around the exhaust outlet. Some industries have put significant provisions on engines with aftertreatment systems that require some type of maintenance at elevated exhaust temperatures. In certain locations, temperatures from exhaust systems must be maintained lower than a certain threshold for safety reasons. These systems have to be taken out of service and moved to special locations for monitoring during the elevated exhaust temperature maintenance activity.
Mechanical diffusers have been designed to help mix fresh ambient air with the exhaust air to reduce the temperature of the exhaust. The mixing effectiveness of the air with the exhaust gases is determined by the exhaust flow through the device, hindering the ability to reliably reduce exhaust outlet temperatures below certain exhaust flow thresholds. In addition, mechanical diffusers increase back pressure on the engine, increasing pumping losses and negatively impacting fuel economy. While larger diffusers may mitigate some of these problems, large diffusers increase the footprint and difficulties with packaging.
Other systems cool the exhaust with intake air from the engine, or by injecting air upstream of the aftertreatment systems. The ability to control the exhaust gas temperature in such systems is dependent on the engine operations or the temperature requirements of the aftertreatment system, also hindering the ability to reliably reduce exhaust outlet temperatures below certain thresholds. Therefore, further improvements in controlling exhaust outlet temperatures are needed.